Comment on acp-2021-820

<strong class="journal-contentHeaderColor">Abstract.</strong> Here we use satellite observations of formaldehyde (HCHO) vertical column densities (VCD) from the TROPOspheric Monitoring Instrument (TROPOMI), aircraft measurements, combined with a nested regional chemical transport model (GEOS-Chem at <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">0.5</mn><mo>×</mo><mn mathvariant="normal">0.625</mn><msup><mi/><mo>∘</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="60pt" height="11pt" class="svg-formula" dspmath="mathimg" md5hash="b5bd9fa9b9f716ab33f0bb6532f347fc"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-7163-2022-ie00001.svg" width="60pt" height="11pt" src="acp-22-7163-2022-ie00001.png"/></svg:svg></span></span> resolution), to better understand the variability and sources of summertime HCHO in Alaska. We first evaluate GEOS-Chem with <i>in-situ</i> airborne measurements during the Atmospheric Tomography Mission 1 (ATom-1) aircraft campaign. We show reasonable agreement between observed and modeled HCHO, isoprene, monoterpenes and the sum of methyl vinyl ketone and methacrolein (MVK<span class="inline-formula">+</span>MACR) in the continental boundary layer. In particular, HCHO profiles show spatial homogeneity in Alaska, suggesting a minor contribution of biogenic emissions to HCHO VCD. We further examine the TROPOMI HCHO product in Alaska in summer, reprocessed by GEOS-Chem model output for a priori profiles and shape factors. For years with low wildfire activity (e.g., 2018), we find that HCHO VCDs are largely dominated by background HCHO (58 %–71 %), with minor contributions from wildfires (20 %–32 %) and biogenic VOC emissions (8 %–10 %). For years with intense wildfires (e.g., 2019), summertime HCHO VCD is dominated by wildfire emissions (50 %–72 %), with minor contributions from background (22 %–41 %) and biogenic VOCs (6 %–10 %). In particular, the model indicates a major contribution of wildfires from direct emissions of HCHO, instead of secondary production of HCHO from oxidation of larger VOCs. We find that the column contributed by biogenic VOC is often small and below the TROPOMI detection limit, in part due to the slow HCHO production from isoprene oxidation under low NO<span class="inline-formula">_<i>x</i></span> conditions. This work highlights challenges for quantifying HCHO and its precursors in remote pristine regions.